192
J. Phys. Chem. 1982, 86, 192-196
Intramolecular Hydrogen Bonding Probed by Laser-Induced Fluorescence. 1. 1,4-Dihydroxyanthraquinone (Quinizarin) T. P. Carter,§ 0. D. Gllllsple,'t and M. A. Connolly5 DepaHmnt of Chembtty, State Universw of New York at Albany, Albany, New York 12222 (Recelved: August 7, 1981)
The fluorescence and fluorescence excitation spectra of quinizarin-doand the fluorescence spectra of quinizarin-dl and -d2 have been measured in n-heptane Shpol'skii matrices at 12 K. Deuterium isotope effects on the fluorescence are small. The geometries of the So and S1states are judged to be similar based on strong (0) bands and a qualitative mirror image symmetry of the fluorescence and excitation spectra. There is no evidence of excited-state proton transfer.
Introduction Intramolecular hydrogen bonds are known to have a considerable effect on the geometric, electronic, vibrational, and radiationless transition properties of substituted aromatic molecules. In addition to the obvious relevance of such effects to the question of conformation in biomolecules (e.g., base pairing in DNA), there are other practical aspects worthy of consideration. The increased photostability conferred on certain molecules by substitution leading to intramolecular hydrogen bonds1q2has been exploited in the development of UV stabilizers. It may be that the enhanced photostability is related to rapid radiationless transitions arising from excited-state transfer of the hydrogen bond proton. More recently there have been studies of the photochemical hole burning (PHB) process in molecules with intramolecular hydrogen bond^;^^^ again, excited-state proton transfer has been suggested as important. The possible application of the PHB process in molecular memories is one motivation for further investigation into the topic. In spite of intense scrutiny by many different experimental techniques (especially IR spectroscopy, NMR spectroscopy, and X-ray diffraction),s there is still much to be learned and understood about intramolecular hydrogen bonds. Moreover, all of the above approaches are limited to the ground electronic state and reveal nothing about the excited electronic state properties. Clearly some form of optical spectroscopy is required for the study of excited-state behavior. Because one ultimately wishes to know the excited-state potential function for the hydrogen bond, it is desirable that the optical experiments be carried out in such a way that the vibronic structure of the electronic transitions is well resolved. The standard approach is to measure high resolution luminescence and luminescence excitation spectra in frozen matrices at low temperature. The results of such experiments, in conjunction with other spectral data, normal coordinate analyses, and electronic structure calculations, should ultimately yield a reasonably complete and detailed description of the intramolecular hydrogen bonds suitable for an understanding of the dynamical role of the hydrogen bonds in various photochemical and photophysical processes. A large fraction of previous work along these lines is represented by the efforts of Brus, Rendpis, Haddon, and co-workers at Bell Laboratories. Among the intramolecularly hydrogen bonded molecules they have studied are methyl salicylate,Btropolone,' 2-(2-hydroxyphenyl)benzo$Department of Chemistry, Iowa State University, Ames, Iowa. Also Department of Physics and author to whom correspondence should be directed.
thiazole,* 9-hydro~yphenalenone,~and 2-methyl-9hydroxyphenalenone.'O In our own work we have concentrated on the spectroscopy of amino- and hydroxysubstituted anthraquinones; one attraction of this class is that one can systematicallyinvestigate the influence of the chemical nature of the proton donor to the hydrogen bond as well as how the properties of the hydrogen bond depend on the position of substitution of the proton donor. For example, consider the following compounds: l-hydroxyanthraquinone, 2-hydroxyanthraquinone, 1,Cdihydroxyanthraquinone, 1,5-dihydroxyanthraquinone,and 1,8-dihydroxyanthraquinone, as well as the series of the corresponding amino derivatives. The kinds of comparisons to be made are fairly obvious. In this paper we present our results for 1P-dihydroxyanthraquinone (quinizarin). The luminescence experiments have been performed in n-heptane Shpol'skii matrices at 12 K with a narrow band tunable dye laser excitation source. In addition to the fluorescence and fluorescence excitation spectra of the normal isotopic species, we have also succeeded in measuring fluorescence spectra for the isomers of quinizarin in which either one or both hydroxy protons have been deuterated.
Experimental Section Dilute solutions (10 pM or less) of quinizarin in n-heptane were frozen to ca. 12 K in a closed cycle refrigerator system (Lake Shore Cryotronics Model LTS-DRC7C). The cooling time from room temperature to 12 K is approximately 1 h. The cell in which the solutions were frozen gives very efficient cooling of the Shpol'skii matrix samples; details of its construction are given elsewhere." The fluorescence was excited with the output of a nitrogen laser pumped tunable dye laser (NRG Model 0.55-150 nitrogen laser and Model DL-0.03 dye laser) operated at 30 or 60 Hz with Coumarin 500 dye (Exciton). The (1)R. Pater, J. Heterocycl. Chem. 7, 1113 (1970). (2)J. R. Merrill and R. G. Bennett, J. Chem. Phys., 43,1410 (1965). Hong, A. Nazzal, and D. Haarer, Chem. Phys. Lett., (3)F.Graf, H.-K. 59., 217 (1978). (4)F.'Dr&ler, F. Graf, and D. Haarer, J. Chem. Phys. 72,4996(1980). ( 5 ) See, for example, M. D. Joesten and L. J. Schaad, "Hydrogen Bonding", Marcel Dekker, New York, 1974. (6)J. Goodman and L. E. Brus, J.Am. Chem. SOC.,100,7472(1978). (7)R. Rossetti and L. E. Brus, J. Chem. Phys., 73,1546 (1980). (8)P. F.Barbara, L. E. Brus, and P. M. Rentzepis, J.Am. Chem. SOC., 102, 5632 (1980). (9)R. Rossetti, R. C. Haddon, and L. E. B m , J. Am. Chem. Soc., 102, 6913 (1980). (10)R. Rossetti, R. Rayford, R. C. Haddon, and L. E. Brus, J. Am. Chem. SOC.,103,4303 (1981). (11)T . P.Carter and G. D. Gillispie, "Shpol'skii Matrix Luminescence Studies with a Closed Cycle Refrigerator," to be submitted. ~~
0022-3654f82f2086-0192~0~ .25fO 0 1982 American Chemical Society
Fluorescence of Quinizarin
bandwidth of the dye laser is approximately 0.04 nm. In some experiments it was found desirable to reduce dye superradiance by passing the dye laser output through a quartz prism monochromator or small grating double monochromator. Two different emission monochromators were used during the course of this work. Most of the spectra were acquired using a 0.45-m grating monochromator (McKee-Pedersen Model 1018B) with a 1200 line/" grating blazed at 500 nm. The limiting resolution of the 1018B is 0.1 nm (ca. 4 cm-' at 500 nm) and we found the width of the vibronic bands to be instrument limited. More recently, we have been using a 1-m scanning monochromator (Spex Model 1802), again with a 1200 lines/" grating blazed at 500 nm. The limiting resolution of the 1802 in first order is less than 0.02 nm. The wavelength readout of neither monochromator was calibrated more accurately than that which can be obtained with a low-pressure Hg lamp. Consequently, the vibronic analyses are good to an accuracy of only about 5 cm-'. For all experiments the emission photomultiplier tube was an EM1 9813 high gain bialkali tube. Signal processing of the PMT output was achieved with a PAR boxcar averager (Model 162 mainframe and Model 164 gated integrator). The output of the boxcar was displayed on a strip chart recorder. The NFLG dye laser employs a micrometer to rotate the echelle grating and hence to determine the wavelength of the dye laser output. To continuously scan the dye laser for the excitation spectra, we used a Spex Model 1673 Mini-drive controller. Mounted on the rotating shaft of the Mini-drive is a gear, over the teeth of which was tightly stretched an O-ring. The micrometer was driven by bringing the O-ring into firm contact with the micrometer head. The dye laser output was scanned at a nominal rate of 0.5 nm/min. The actual wavelengths of the excitation features were determined by hand tuning the dye laser to optimize the emission signal and measuring the wavelength with the emission monochromator.
Results and Discussion Fluorescence of Quinizarin-do. There have been a few previous reports of high-resolution laser excited fluorescence spectra of quinizarin in Shpol'skii matrices. Hong et al.3 and later Drissler, Graf, and Haarer4 (DGH) presented the 2 K fluorescence spectrum in n-heptane with Ar+ laser excitation obtained as part of their investigation of photochemical hole burning. Vibrational frequencies were assigned to seven ground state fundamentals: and a brief discussion of the vibronic nature of the spectrum was given. It was stated that there are two prominent sites in the n-heptane matrix. DGH also suggested that quinizarin could be thought of as a donor-acceptor complex, with 1,4-naphthoquinone functioning as the acceptor and 1,4hydroquinone as the donor. In a recent paper, Anoshin, Gastilovich, and Shigorid2(AGS) reported liquid helium temperature fluorescence spectra of quinizarin and quinizarin-d, (in which both hydroxy protons are deuterated) in n-octane Shpol'skii matrices. From vibrational analyses of the spectra and normal mode calculations based on a force field deduced for 1,4,5,8-tetrahydroxyanthraquinone,13AGS conclude that the electronic excitation in quinizarin is localized in the quasi-aromatic rings containing the intramolecular hydrogen bonds. (12)A. N. Anoshin, E. A. Gastilovich, and D. N. Shigorin, R u s . J. Phys. Chem., 54, 1409 (1980). (13)E.A. Gastilovich, L. V. Golitayna, G. T. Kryuchkova, and D. N. Shigorin, Opt. Spektrosk., 40,800 (1976).
The Journal of Physical Chemistry, Vol. 86, No. 2, 1982 193
EMISSION WAVELENGTH [ NM)
Figure 1. Fluorescence spectrum of quinizarin-d, in nheptane at 12 K. Labels on bands correspond to those given in Table I. Ordinate is relative intenslty and is uncorrected for detector response. The wavelength of the (0-0) band is 522.7 nm and its intenslty is approxlmetely four times that of the strongest vibronic feature (band 4). This spectrum was taken with the Spex 1802 emission monochromator.
In all of these previous studies fixed frequency Ar+ laser excitation at 488.1 nm was used. In our work we have used a tunable dye laser as the excitation source in order to ensure that we obtain the single-site spectra best suited for vibrational analysis. Also, by varying the wavelength of excitation, we obtain the active fundamentals for the S1 S,transition and additional evidence on the geometry differences between So and S1. In Figure 1we show the fluorescence spectrum of quinizarin in an n-heptane Shpol'skii matrix at approximately 12 K. This spectrum was excited at a wavelength of 508.7 nm, or 525 cm-' above the prominent origin band at 522.7 nm. Let us first comment on the intensity of the (0-0) band relative to that of the other strong vibronic bands in the spectrum. Whereas in the work of DGH and AGS the origin band was not the strongest feature in the spectrum, we find that it is about four times stronger than the most intense fundamental located 459 cm-' from the origin. We must conclude that in the previous studies the origin band was severly attenuated by reabsorption. The fact that the (0-0)band is so intense is a strong indication that the S1-& transition is allowed and there is not a large geometry change between the upper and lower electronic states. It appears that the higher temperature of our experiments compared to those in the work of DGH (12 K vs. 2 K) has not resulted in any appreciable broadening of the spectral features. We measure the bandwidth of the (0-0)band to be 2.5 cm-l at FWHM. This is to be compared with the 8 cm-l bandwidth quoted by AGS for the n-octane matrix at 4.2 K. Also note the structured and partially resolved features about 20 cm-I to the red of the origin. These appear to correspond to what was assigned as a second site by DGH. Because this structure is unaltered with changes in excitation wavelength, even for direct excitation into the origin band, there is no doubt that our spectrum is that of a single site. There is also a weak feature to the blue of the origin band which probably represents a transition from a thermally populated phonon level (vide infra). The vibrational analysis of the quinizarin fluorescence spectrum is given in Table I and is in good agreement with that of AGS. The only deviation for fundamentals of frequency less than 1425 cm-' is that we find a very weak one at 1344 cm-I not observed by AGS. They also assign an additional four fundamentals between 1439 and 1654 cm-' which we either assign as combination bands or do not observe. Given the difference in solvents, excitation sources, and bandwidths in the experiments, the harmony in the analyses is highly satisfactory. The list of seven fluorescence active fundamentals given by DGH includes one at 1681 cm-l, which is coincidentally the frequency of the carbonyl stretching vibration in anthraq~in0ne.l~The intramolecular hydrogen bonds between the hydroxy
-
(14)C. Pecile and B. Lunelli, J. Chem. Phys., 46,2109 (1967).
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The Journal of Physical Chemistry, Vol. 86, No. 2, 1982
Carter et al.
TABLE I: Vibrational Analysis of the Fluorescence Spectrum of Quinizarin-do in n-Heptane a t 1 2 K band no.
AT,
cm-I
(f
0
-
1 2 3 4 5 6 7 8 9 10
312 421 437 459 513 542 626 653 735 773 841 855 876 880 895 919 973 1073 1115 1148 1162 1189 1228 1239 1268 1295 1309 1334 1344 1410 1491 1531 1541 1551 1592 1621 1647 1658 1687 1698 1723 1742 1829 1866
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
re1 intensityb -400
-
40 70 15
100
5 2 2 6 6 7 4 3 23 shoulder
